1. Field of the Invention
This invention relates to an exhaust purification apparatus for an engine and, more particularly, to an exhaust purification apparatus for an engine in which a catalytic converter for reducing noxious ingredients included in exhaust gas by catalysis is located in an exhaust passage on a downstream side of the engine combustion chambers, and in which the air fuel ratio of mixture supplied to the combustion chambers is controlled, by feedback, towards a target air fuel ratio in response to a detected value of an exhaust sensor disposed on an upstream side of the converter for detecting concentration of remaining oxygen in the exhaust gas.
2. Description of the Related Art
There is known for an engine for a vehicle or the like, a catalytic converter which, in order to reduce noxious ingredients included in exhaust gas, uses a three element catalyst for the purification of three ingredients, namely carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx). The converter is located in an exhaust system in which air fuel ratio control is performed so as to keep the air fuel ratio of the mixture supplied to the combustion chambers to a predetermined target air fuel ratio (for example the theoretic air fuel ratio; air/fuel = 14.7) so that the catalysis of the three element catalyst is effectively obtained. More specifically, the air fuel ratio control is performed by an exhaust sensor, in which an output condition is changed with respect to a condition that air surplus ratio LAMBDA (actual air fuel ratio/theoretic air fuel ratio) is equal to 1, as a border, on an upstream side of the converter, and either decreasing fuel supply volume when a detected value of the exhaust sensor indicates a state of oxygen shortage (a state wherein, for example, fuel is in surplus against air; hereinafter, called a rich state) or increasing fuel supply volume when the detected value indicates an oxygen surplus state (a state wherein, for example, fuel is thin against air; hereinafter, called a lean state), so that the air fuel ratio of mixture supplied to the combustion chambers converges towards the target air fuel ratio.
The three element catalyst used for the catalytic converter may cause a problem in that deterioration of performance occurs over a period of time, because of attachment of impurities to the catalyst ingredients from, for example, leaded gasoline.
To overcome such a problem, there has been proposed an exhaust purification apparatus, in which a second exhaust sensor for detecting oxygen concentration is disposed on a downstream side of the catalytic converter, and in which deterioration of catalysis is judged by inverting the output of the exhaust sensor at the time of feedback control of air fuel ratio, see for example, Japanese Unexamined Patent Publication No. 63-97,852. In this apparatus, deterioration of the catalyst is indirectly judged, by providing that no difference of concentration of oxygen is produced between the downstream side of the catalyst and the upstream side, due to a decline of performance of oxygen storage at the time of deterioration of the catalyst.
Meanwhile, since the catalyst in the catalytic converter performs to repeat oxidation-deoxidation reactions against the noxious ingredients by absorbing and dissociating of oxygen, problems may occur in that a partial pressure of oxygen on the downstream side of the catalytic converter is not prone to change and that it is not defined in either the oxygen surplus state or the oxygen shortage state related to a driving condition. Namely, since the mixture supplied to the combustion chambers of the engine is controlled to within the vicinity of the theoretic air fuel ratio, oxygen concentration in the exhaust gas exhausted from the combustion chamber becomes a value reflected in the theoretic air fuel ratio. Therefore, if oxygen quantity in the exhaust gas on the upstream side of the catalytic converter is high, oxygen on the downstream side of the converter is in a surplus state by oxygen overflown from performance of oxygen storage of the catalyst. Alternatively, if oxygen quantity in the exhaust gas is low, since all oxygen is absorbed in the catalyst, oxygen on the downstream side is in a shortage state. Consequently, in the case that deterioration judgement of the catalyst is performed on the basis of a detected value of the exhaust sensor on the downstream side of the converter, a problem occurs in that an adequate judgement accuracy is not obtained.
Against such problems, although it is thought that control constant of the feedback control should be changed so that a partial pressure of oxygen on the downstream side of the catalytic converter around the exhaust sensor is shifted to either the oxygen surplus state (the lean state) or the oxygen shortage state (the rich state) during detecting deterioration of the catalyst, a new problem may occur in that the control constant is not easily determined due to difference of system structures of engines, disunity of output characters of exhaust sensors, change in a long time and so on. Namely, even if the control constant is set so that, for example, the exhaust state during detecting deterioration shifts to the lean state, if it is shifted to a state which is too lean, reliable detecting sensitivity is not obtained because the output of the exhaust sensor does not tend to invert to the rich state, and if it is not shifted, accuracy of the judgement is not improved.
Moreover, since the catalyst of the catalytic converter in an initial period of use has an excellent performance of oxygen storage in particular, another problem may occur in that variation of oxygen concentration on the downstream side of the catalytic converter is small by an ordinary air fuel ratio feedback control and so that a result of detection of the exhaust sensor does not reflect an active state of the catalyst.
Furthermore, the exhaust sensor used for detecting deterioration of the catalyst is itself deteriorated during a long period of use. Even if a control constant of feed back control for detecting deterioration of the catalyst set at an initial period is used, by a decline of character of reaction of oxygen of the exhaust sensor, even if activation of the catalyst is high enough, a misjudgment that the catalyst is in a deteriorated state may occur.